If The Dark Knight is the greatest superhero movie of all time—and I think it probably is—it is entirely on the back of Heath Ledger's immaculate turn as the Joker. Ledger is utterly, utterly, utterly perfect in this role—so perfect in fact that it is impossible to imagine either anyone else ever playing the Joker or any other villain stepping in to carry a sequel.
(Who's even still on the bench at this point? The Penguin? Riddler? King Tut? Catwoman can't carry a movie all by herself. The best bet, it seems to me, would be to go forward with the long-teased Batman vs. Superman project; it'd be something of a genre mismatch for the Christian Bale franchise, but at least it'd spare us all another round of movie nonsense with the highly overrated Riddler.)
There's no question about it: Ledger's performance is simply stunning. His sociopathic Joker is so good that it's hard to say that the movie is actually enjoyable to watch—I feel exhausted after seeing it, disturbed and just a little bit broken. In this sense The Dark Knight isn't really a superhero movie at all, but a horror movie, a slasher flick, and really—with Ledger so famously dead
by suicide* just after shooting—a snuff film. As David Denby put the point in the New Yorker:
When Ledger wields a knife, he is thoroughly terrifying (do not, despite the PG-13 rating, bring the children), and, as you’re watching him, you can’t help wondering—in a response that admittedly lies outside film criticism—how badly he messed himself up in order to play the role this way. His performance is a heroic, unsettling final act: this young actor looked into the abyss.This is in all respects an astoundingly dark movie that's hardly suitable for adults, much less children. And if The Dark Knight doesn't quite possess the necessary sense of self-awareness to be the Watchmen of superhero film, it may well be its Dark Knight Returns (wiki)—or, perhaps more directly, the story from which it draws its most direct inspiration, Alan Moore's definitive Joker story, The Killing Joke (wiki).
What Moore gets, of course, is what the film is only able to hint at: the extent to which Batman and the Joker (to mix my supervillain metaphors) are two sides of the same coin. It is not just that they are both insane, but that they are both equally insane and insane in exactly the same way—just in opposite directions.
* Abe rightly points out in the comments that Ledger's death by overdose probably wasn't a suicide. Obviously I'm not watching enough entertainment television.